The present invention relates to a pressure sensor using the piezoresistivity effect of a semiconductor material, and more particularly to a pressure sensor having a silicon diaphragm, on one surface of which a diffused resistor is formed as a pressure-sensitive element which converts a fluid pressure applied to the diaphragm to a corresponding electrical signal, by sensing changes in the resistance of the diffused resistor due to stress occurring therein.
This type of sensor is relatively small and of good sensitivity. These sensors can easily be manufactured in large quantities and to high standards of uniformity using integrated circuit manufacturing techniques. These excellent features have received engineering attention, and research in their practical use has been done in many factories and research institutes. One of the problems to be solved with this sensor is its thermal characteristics. That is, if the silicon diaphragm and a base to which the silicon diaphragm is secured have different linear coefficients of thermal expansion, thermal stresses will occur in the diaphragm due to a change in its temperature, and thus the output of the sensor will fluctuate. In order to avoid the influence of such thermal change, materials whose linear coefficient of thermal expansion is very close to that of silicon have been used for the mounting base; for example, mullite, zircon, and Pyrex glass (trade mark). However, the above-mentioned materials for the diaphragm and its mounting base, respectively, still have nonuniformities in linear coefficient of thermal expansion, which are difficult to remove completely. When the mounting base used is made of a ceramic material such as mullite, zircon or Pyrex glass, the bonding layer between the base and the diaphragm must be relatively thick because of the roughness of the surface of the base. Thus the diaphragm is still susceptible to thermal expansion of the bonding layer and therefore this technique has not succeeded in sufficiently reducing thermal stress in the diaphragm.
Other proposals for improving the thermal characteristics of the diaphragm have been made which involve mounting the diaphragm on a silicon support plate having the same linear coefficient of thermal expansion as the diaphragm. FIG. 1 of the accompanying drawings shows one such proposal where a pressure sensor 10 includes a diaphragm assembly 12 which consists of a silicon diaphragm block 14 having a diaphragm 16 on which diffused resistors 18 are formed and a silicon support plate 20 bonded to the opposite surface of the diaphragm block 14 by an Au-Si (gold-silicon) eutectic alloy layer (not shown), an alumina base 22 to which the diaphragm assembly 12 is sealingly bonded at its center on its back surface by a bonding material 24. The space 26 between the diaphragm block and the support plate is evacuated. The surface of the diaphragm on which the diffused resistors 18 are formed is covered by a cap 28 secured to the base. A fluid pressure subject to measurement is introduced through an inlet pipe 30 and a hole 32 provided in the base 22 and transmitted to the diaphragm 16 and therefore the diffused resistors 18.
In this sensor, the diaphragm and the support plate are made of a material of the same linear coefficient of thermal expansion, a single crystal silicon, and therefore the diaphragm is less susceptible to thermal stress than if the diaphragm were bonded to a mounting base of a material of different linear coefficient of thermal expansion. However, since the diffused resistors are exposed to a fluid whose pressure is subject to measurement, they are liable to deteriorate due to moisture and corrosive gases possibly contained in the fluid. Thus, this sensor is not suitable for use in an oxidizing atmosphere discharged from an internal combustion engine for the purpose of sensing the pressure thereof.
Another prior art pressure sensor 10 shown in FIG. 2 includes a silicon diaphragm block 14 bonded to a silicon support plate 20 which is formed integrally with a support pillar 34 which in turn is firmly threaded in an alumina base 22. A fluid pressure subject to measurement is introduced through an inlet pipe 30 and a passage 36 provided in the pillar 34 so as to arrive at the back of the diaphragm 16. In this sensor, the surface of the diaphragm on which the diffused resistors 18 are formed is exposed to a vacuum within a cap 28 and therefore the diffused resistors 18 are not deteriorated by the atmosphere fluid subject to measurement. Further, use of a relatively long support pillar 34 serves to reduce the transmission to the silicon diaphragm of thermal stress, produced by the difference in linear coefficient of thermal expansion between the support pillar 34 and the base 22. However, in this sensor, a single piece of the support plate and the pillar must be cut from a single crystal of silicon material, thereby producing a considerable amount of scrap; this is not suitable for mass production. Reference numeral 38 denotes terminals connected through wires 40 to the diffused resistors 18, respectively.